The invention relates to a glass melting tank and a process for melting glass. A preferred application is the refining of glass melts.
Glass melting tanks have been known for a long time, in particular, glass melting tanks that can be operated continuously were a precondition for the rise of large enterprises in the field of glass with industrial series production.
Here tanks are distinguished especially according to their construction type, their size, and their heating system.
The heating of glass melting tanks by direct electrical resistance heating, i.e., by conducting an electrical current through the glass melt by electrodes dipping into it, is an increasingly used process, since high efficiencies are reached in terms of thermal engineering. Fully electrical heating and supplemental electrical heating are found.
Usual materials for such electrodes are molybdenum (Mo), wolfram (W), platinum (Pt) and also zinc oxide (SnO2).
In melting glass under oxidizing conditions or in melts that contain substances that are easily reduced, i.e. e.g., in lead-containing or arsenic- or antimony-containing glasses, corrosion of the electrode material, in particular of molybdenum, especially at high temperatures of the glass melt in contact with the electrodes, is a big problem.
Thus, e.g., a glass melting tank with an electrical heating system is known from U.S. Pat. No. 4,819,247 in which the heating electrodes and the corresponding counter-electrodes project into the melt from opposite lateral walls.
A process and a device, in particular a refining tank for melting glass, are known from EP 0 393 882 A2. The refining tank, made flat, can be heated electrically by electrodes that project into the melt from the lateral walls, in addition the tank has burners above the melt.
After the end of the rough melt, there remain in the melt, in addition to large amounts of dissolved gases, so many bubbles that the glass would be unusable. The object of refining is to remove the remaining bubbles, to reduce the concentration of dissolved gasses that could give rise to after generation of gas, and to homogenize the melt. Glass technology uses thermal, chemical, and mechanical aids or a combination of them for this purpose.
To support the refining thermally, the temperature of the melt is raised as high as possible to achieve higher partial pressures of the refining gas and thus an enlargement of the bubbles as well as, by reducing the viscosity of the melt, a more rapid elimination of the bubbles from the melt.
But countering the desire for a higher temperature of the melt, especially in the refining area of the tank, is the corrosion of the heating electrodes in the mentioned electrical heating systems, which is already a problem anyway. With increasing temperature of the melt, increasing electrode corrosion can be observed, since the electrodes project directly into the hot melt. Likewise problematic is the increasing corrosion of the tank as the temperature of the melt increases.
An object of the invention is to find a glass melting tank and a process for melting glass in which, by direct electrical heating of the melt, as high a temperature as possible of the melt, in particular in the refining area of the glass melting tank, is achieved and corrosion of the heating electrodes as well as of the glass melting tank can be kept slight.
Further, the tank and the process are to be able to be performed simply and with low investment costs using slight modification of already existing tanks and processes.
The glass melting tank has, at least in the area of the melt, a narrowed cross section area, and the glass melting tank has at least one heating electrode in front of and one heating electrode behind the narrowed cross section area and in this way it is possible to achieve an increase in the temperature of the melt in the narrowed cross section area.
In the process according to the invention for melting glass, the melt is introduced into a glass melting tank with a narrowed cross section area at least in the area of the melt, and the narrowed cross section area is heated by at least one heating electrode in front of and a corresponding heating electrode behind the narrowed cross section area and thus a zone of increased temperature of the melt is formed in the narrowed cross section area.
Because the glass melting tank according to the invention has, at least in the area of the melt, a narrowed cross section area and at least one heating electrode for direct electrical heating of the melt in front of, and a corresponding heating electrode behind the narrowed cross section projecting into the melt, it is advantageously possible to achieve very high temperatures of the melt inside the narrowed cross section area and simultaneously to keep electrode corrosion slight. The electrodes are located in the less hot area of the melt in front of and behind the narrowing.
The delivery of the heating current by the electrodes is performed in an area of the glass melting tank in which the temperatures of the melt that were usual up to now occur, so that electrode corrosion does not exceed the extent known up to now. The narrowed cross section area of the tank between the heating electrodes causes the flow lines to be pulled together. In this way, greater flow density and thus a higher temperature of the melt, locally separate from the location of the electrodes, are achieved in this area.
The respective size and geometry of the cross section narrowing has a considerable influence on the temperatures that can be achieved in the narrowed cross section area.
In a glass melting tank according to the invention that has a given narrowed cross section area, the temperature of the melt in the narrowed area can be adjusted and controlled essentially by the electrodes and, for example depending on the type and number of electrodes or on the type and magnitude of the operating voltage and of the current, a temperature adjustment and control can take place.
Thus, e.g., a temperature of the melt in the narrowed cross section area of up to 2000xc2x0 C., and preferably of up to 1800xc2x0 C., can be set. Here it is possible with difficulty, by heating the melt only with gas burners, to achieve the indicated high temperatures in the glass melting tank.
Use of heating electrodes in an area of the melt having the temperatures indicated would lead to a great increase in electrode corrosion because of the low viscosity of the melt.
The narrowed cross section area is preferably made by an inward arching of the lateral surface or by an inward arching of two opposite lateral surfaces of the glass melting tank.
Preferably the cross section narrowing of the tank is symmetrical, in particular symmetrical with respect to the lengthwise axis of the glass melting tank.
The cross section narrowing can also be constructed by a partial reduction in the tank depth, e.g., by inwardly arching the bottom surface of the tank.
Especially advantageous is a glass melting tank in which the narrowed cross section area is made by symmetrical inward arching of the opposite lateral surfaces as well as by a pronounced inward arching of the bottom surface.
Here it has turned out to be especially advantageous if the narrowed cross section area is up to 15% of the total length of the glass melting tank. Preferably the length of the narrowed cross section area is 150 to 460 cm, its width 40 to 150 cm and its depth 10 to 60 cm.
To keep the increasing tank corrosion slight at high melt temperatures, the wall of the tank is cooled at least in the narrowed cross section area. Preferably the wall here is cooled by water cooling. The cooling of the wall also cools the melt, which is in direct contact with the wall, and thus the corrosion of the wall is kept slight. The hot melt that is further away from the wall has contact only with the less hot melt but no direct contact with the wall.
The glass melting tank can be additionally heated in the narrowed cross section area by burners, in particular air/gas- and/or oxygen/gas burners. This results in a minimum surface temperature of the melt of about 1680xc2x0 C.
The areas of the glass melting tank located in front of and/or behind the narrowed cross section area are preferably heated with electrodes and/or by burners, in particular air/gas and/or oxygen/gas burners.
In an especially preferred embodiment of the glass melting tank, the narrowed area is the refining area. The high temperature of the melt achievable according to the invention is used here essentially for the mentioned support of refining.
Because the part of the refining area located above the melt can be separated in a vapor-proof way from the rest of the tank, for example by partitions that project at least partially from above into the melt, the refining area located above the melt can be at least partially evacuated.
The use of vacuum additionally supports refining, and the low pressures above the melt make possible a rapid growth of bubbles and thus accelerate the rise of the bubbles in the melt that already has low viscosity.
The geometry of the narrowed cross section area is configured advantageously so that the melt has a large surface area relative to its volume, also to accelerate the driving out of bubbles. The ratio of volume to surface area of the melt in the narrowed area is preferably 1 m3 to 4 m2.
In an advantageous configuration of the invention, the electrodes project from below and/or from the lateral surfaces of the glass melting tank into the malt, and the electrodes can be used in the form of block electrodes. The electrodes are optionally cooled.
In principle, all previously cited electrodes can be used, and electrodes made of molybdenum are preferred, since molybdenum is relatively strong, economical and easily machined. Electrodes made of platinum are also used, and platinum is more corrosion resistant against glass melts.
The voltage between the electrodes is preferably up to 1500V, preferably up to 1000V, and the current is up to 7000 A, preferably up to 5000 A.
In principle all electrical heating processes are usable, in particular heating processes in which an alternating voltage with a frequency of up to 20 kHz is applied to the electrodes.
To keep floating, still incompletely dissolved substances on the surface of the glass melt away from the narrowed cross section area, a barrier (flow-through wall) can be placed in front of the narrowed cross section area, under which the melt flows through.